1. Field of the Invention
The present invention relates to a system for purging exhaust gases from an exhaust gas recirculation (EGR) system for a compression-ignition internal combustion engine to minimize corrosion of EGR components caused by condensation of residual gases in the EGR system.
2. Background Art
Compression-ignition internal combustion engines may be equipped with EGR systems to reduce NOX emissions. EGR systems include an EGR circuit in which tubing interconnects an EGR cooler, EGR flowmeter, and EGR valve.
When an engine is operating, hot exhaust gases may be circulated through the EGR system. When the engine is shut down, the components of the EGR system cool causing condensation. The gases that condense in the EGR system after engine shut down are acidic and can cause corrosion of the components of the EGR circuit. As the exhaust gases cool in the EGR circuit, condensation forms on the interior surfaces of the components of the EGR circuit.
EGR systems for diesel engines equipped with a turbocharger that pressurizes intake air require a system for increasing pressure in the EGR system above the pressure of the intake. For example, with a variable geometry turbocharger, the vanes of the turbine can be partially closed to create back pressure to allow flow in the EGR system.
There is a need for a method and apparatus for purging gases from the EGR circuit when the engine is shut down to avoid or minimize condensation of the acidic EGR gases in the EGR circuit. There is also a need to purge EGR gases by flushing with fresh intake air to reduce the acidity of any condensate and prolong the life of the EGR circuit components by minimizing corrosion.
The above problems and needs are addressed by Applicant""s invention as summarized below.
According to one aspect of the present invention, a system for providing exhaust gas recirculation in a multi-cylinder compression-ignition internal combustion engine having an intake side and an exhaust side is provided wherein intake air is ported through the EGR system prior to engine shut down. The system includes an EGR valve in communication with the exhaust side of the engine that selectively diverts a portion of exhaust gases from the internal combustion engine through an EGR circuit to the intake side of the engine. The EGR control can be used to provide higher intake manifold pressure than the exhaust manifold pressure prior to or as part of engine shut down. By creating higher pressure on the intake side while the EGR valve is held open, exhaust gases can be flushed from the EGR circuit. If a variable geometry turbocharger is provided as part of the engine, the turbine vanes may be adjusted to reduce pressure in the EGR circuit and thereby allow the intake manifold pressure to be higher than the exhaust manifold pressure.
According to another aspect of the invention, the EGR valve may be held open by an engine control module for a predetermined period of time after the engine reaches idle condition. The EGR valve may be held open for a predetermined period of time that is at least equal to the period of time required to fill the EGR system three times with air.
According to another aspect of the invention, a method of purging exhaust gases from an EGR system of a multi-cylinder compression-ignition internal combustion engine is provided. The method includes the step during engine shut down of setting the intake manifold pressure higher than the exhaust manifold pressure. The EGR valve is held open for a predetermined period of time so that air may be directed from the intake manifold to the EGR system and into the exhaust manifold.
According to another aspect of the invention, a method is provided for purging exhaust gases from an EGR system of a multi-cylinder compression internal combustion engine that powers a generator set. The engine has an intake side and an exhaust side that runs at light loads for a period of time before shut down while the engine is operating at light loads. The method includes setting the intake manifold pressure higher than the exhaust manifold pressure, opening the EGR valve for a predetermined period of time and directing air from the intake manifold to the exhaust manifold into the EGR system.
According to other aspects of the invention, the method of purging exhaust gases from the EGR system can be utilized on an engine flowing EGR at idle. When power to an ignition circuit is turned off, the method may also be carried out with an engine having a variable geometry turbocharger that may be set to hold the intake manifold pressure higher than the exhaust manifold pressure while the EGR valve is held open at engine shut down.
According to another aspect of the invention, if the intake manifold pressure is not maintained at a higher pressure than the exhaust manifold pressure, then the EGR valve may still be held open during engine spin down to allow exhaust gases (no combustion during engine spin down) to continue to enter the EGR system thereby allowing cleaner air to flush the EGR circuit.
The above advantages, and other advantages, objects, and features of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.